Wireless communication systems are fundamentally limited by the availability of the electromagnetic spectrum in which they must operate. Consequently, increasing spectral efficiency has been a major focus of research over recent decades, and given the exponential growth in demand for radio services, spectral efficiency will continue as a key research driver for years to come. Radio signals attenuate quickly with distance, and therefore in radio systems the transmit signal powers are typically much higher than receive signal powers (often over 100 dB higher in cellular systems). Because of this, it has long been held that a radio system cannot transmit and receive on the same frequency at the same time, as the high powered transmit signal would lead to catastrophic self-interference at the receiver.
In conventional radio systems, duplex operation is achieved by simply avoiding this problem. Spectral resources are divided between the transmit and receive channels either in time, using time division duplexing (TDD), or frequency, using frequency division duplexing (FDD).
In FDD radio operation, there are two separate carriers at different frequencies, one for the uplink transmission and one for the downlink transmission. Isolation between the downlink and uplink transmissions is achieved by transmission/reception filters called duplex filters. Otherwise known as duplexers, these duplex filters are typically implemented as two highly selective filters, one centred at the receive (RX) band, the other centred at the transmit (TX) band to separate the TX and RX signals, thereby preventing the TX signal from interfering with the RX function.
The achievable TX-RX isolation of a duplexer is of primary concern, since a higher duplexer isolation simultaneously relaxes the noise requirements of the transmitter and the linearity plus phase-noise requirements of the receiver. For example, certain cellular radio standards dictate a TX-RX isolation of 52 dB in the TX band and 45 dB in the RX band. These stringent isolation requirements are met in modern duplexers by employing highly selective surface acoustic wave (SAW) filters.
These filters must be implemented off-chip (i.e. they cannot be integrated with a CMOS process) owing to the high-Q resonators used to build the SAW filters. This usually presents no problems for a simple radio transceiver operating on one frequency band. However, modern radio transceivers are usually multiband. For example, there are 38 operating bands for LTE (Long-Term Evolution) as defined by the 11th Release of the Third Generation Partnership Project (3GPP Rel. 11), 26 of which require FDD operation.
This means a separate duplexer is required for each band of operation. The ‘bank’ of discrete duplexers is usually connected to the antenna via a multipole RF switch, which selects the appropriate duplexer based on the frequency band of operation. Not only does this add to the complexity of the device, it also impacts the overall size and cost of the multiband transceiver which relies on a highly integrated solution for economies of scale. Consequently, a duplexing apparatus is highly desired which can not only support multiple bands, but which can also be fully integrated on-chip.